Musnad Abu Ya'la al-Mawsili 129:164

Zuhayr narrated to us, Abdullah bin Bakr narrated to us, Humayd narrated to us, from Thabit, from Anas, from the Prophet (peace be upon him): That he saw an old man being supported between his two sons. They said, "O Messenger of Allah, he vowed to walk." He said, "Indeed, Allah is free of need from this man torturing himself." Then he commanded him, so he rode.